Recently, as power supplies for driving portable electronic equipment, such as cell phones, portable personal computers, and portable music players, and further, as power supplies for hybrid electric vehicles (HEVs) and electric vehicles (EVs), nonaqueous secondary batteries represented by lithium ion secondary batteries having a high energy density and high capacity are widely used.
As for the positive electrode active material in these nonaqueous secondary batteries, one of or a mixture of a plurality of lithium transition-metal composite oxides represented by LiMO2 (where M is at least one of Co, Ni, and Mn), (namely, LiCoO2, LiNiO2, LiNiyCo1-yO2 (y=0.01 to 0.99), LiMnO2, LiMn2O4, LiCoxMnyNizO2 (x+y+z=1)), phosphoric acid compounds having an olivine structure such as LiFePO4, and the like, all of which can reversibly absorb and desorb lithium ions, is used.
Carbonaceous materials such as graphite and amorphous carbon are widely used as the negative electrode active material in nonaqueous secondary batteries. The reason is that carbonaceous materials have a discharge potential equal to that of metal lithium or a lithium alloy but do not cause dendrite growth, and thus carbonaceous materials have superior characteristics of high safety, superior initial efficiency, good potential flatness, and high density.
Carbonate esters which are also referred to as carbonates, lactones, ethers, esters, and the like are used alone or in mixtures of two or more as the nonaqueous solvent for a nonaqueous electrolyte. Among them, carbonate esters are widely used because they have an especially high dielectric constant and provide larger ion conductivity to the nonaqueous electrolyte. As the nonaqueous solvent, JP-A-2008-084705 discloses that, when a DOX derivative is added to a mixed solvent containing both ethylene carbonate (EC) and propylene carbonate (PC), oxidative decomposition of the nonaqueous electrolyte is suppressed, the nonaqueous electrolyte is electrochemically stabilized, and thus cycle characteristics are improved.
JP-A-09-199112 discloses an example in which a positive electrode mixture is mixed with an aluminum coupling agent in order to improve cycle characteristics when a nonaqueous secondary battery is charged and discharged at high voltage. Furthermore, JP-A-2002-319405 discloses an example in which a silane coupling agent having an organic reactive group such as an epoxy group and amino group and a bonding group such as a methoxy group and ethoxy group is dispersed in a positive electrode mixture in order to improve wettability of a positive electrode with an electrolyte in a nonaqueous secondary battery at low temperature and to improve output characteristics at low temperature.
JP-A-2007-242303 discloses an example in which a positive electrode active material is treated with a silane coupling agent having a plurality of bonding groups in order to improve cycle characteristics when intermittent cycles of a nonaqueous secondary battery are repeated. JP-A-2007-280830 discloses an example in which a silane coupling agent is present near a broken surface of a positive electrode active material occurring when a positive electrode mixture layer is compressed in order to improve cycle characteristics of a nonaqueous secondary battery. Furthermore, JP-A-2007-305453 discloses an example in which a slurry of an electrode active material binder is mixed with a surface treating agent and an electrode active material is surface-treated.
In the invention disclosed in JP-A-2008-084705, it is clear that cycle characteristics are improved because the nonaqueous electrolyte contains a DOX derivative. However, there is problem that when a nonaqueous electrolyte containing a DOX derivative is used, self-discharge increases after repetition of charge and discharge cycles at high temperature.
The inventions disclosed in JP-A-09-199112, JP-A-2002-319405, JP-A-2007-242303, JP-A-2007-280830, and JP-A-2007-305453 show that mixing a silane or aluminum coupling agent in a positive electrode mixture can possibly lead to an improvement in cycle characteristics and output characteristics in a low temperature environment to some extent. However, in JP-A-09-199112, JP-A-2002-319405, JP-A-2007-242303, JP-A-2007-280830, and JP-A-2007-305453, effects by adding a DOX derivative into a nonaqueous electrolyte is not described when the positive electrode mixture is mixed with a silane or aluminum coupling agent.
The inventors of the present invention have carried out various studies in order to suppress the increase of self-discharge after repetition of charge and discharge cycles at high temperature when such a nonaqueous electrolyte containing at least a DOX derivative as a nonaqueous solvent in the nonaqueous electrolyte is used. As a result, the inventors have found that the problems mentioned above can be solved when a positive electrode mixture contains a predetermined amount of a silane or aluminum coupling agent and the average particle diameter and the specific surface area of a positive electrode active material including a lithium composite oxide are maintained in a predetermined range, whereby the invention has been achieved.